NMR and luminescence studies on the formation of ternary adducts between HSA and Ln(III)-malonate complexes (Ln=Eu, Gd, Tb)

At physiological pH and in the presence of an excess of malonate ligand (MAL), the lanthanide ions (Ln=Eu(III), Gd(III) and Tb(III)) are under the form of [Ln(MAL)2(H2O)4]-. Upon addition of human serum albumin (HSA), formation of two different ternary adducts of stoichiometry HSA-Ln(MAL)(x)(H2O)(q) (x=2, q=2; x=2, q=4) is detected. On the basis of the reasonable assumption that the binding strength for the two sites on the protein are inversely proportional to the hydration state of the metal ion, stability constants of 4.0.103 M-1 and 3.5.102 M-1 have been evaluated for the system with q=2 and q=4, respectively. Whereas for the stronger binding site it is suggested that the protein provides two or three donor atoms to the coordination cage of the Ln(III) ion, in the case of the weaker binding site it is likely that it corresponds to a simple electrostatic interaction between the negatively charged [Ln(MAL)2(H2O)4]- and positively charged groups on the surface of the protein.